Conventionally, as a device for detecting an air pressure in a vehicular tire, there has been proposed a direct detection device which uses a pressure responsive member within the tire. However, because the pressure responsive member must be provided within the tire, this device results in complicated construction and high costs.
Therefore, there has been proposed a device for indirectly detecting the tire air pressure which is based on a detection signal from a wheel speed sensor. This detection signal represents a wheel speed for each wheel which is based on a relationship between the tire radius and the tire air pressure. For instance, when the tire radius becomes smaller, the tire air pressure is decreased.
The tire radius may be affected by differences in each tire which are due to tire wear or traveling conditions such as cornering, braking, starting or so forth. Furthermore, the radius of many tires does not change responsively to changes in the tire air pressure. For instance, when the tire pressure is decreased at a rate of 1 Kg/cm.sup.2, the corresponding tire radius deformation magnitude may be only 1 mm. For these reasons, the method for indirectly detecting the tire air pressure based on the deformation magnitude in the tire radius, is problematic in that it cannot always provide accurate tire air pressure detection.